1. Field of the Invention
The present invention relates to an optical head irradiating an optical disk with a light beam to record or reproduce information, particularly an optical head comprising a relay lens to correct spherical aberrations, and to an optical disk apparatus comprising such an optical head.
2. Description of the Related Art
In recent years, in the field of recording or reproduction using, for example, an optical disk as a recording medium, many attempts have been made to an optical disk recording and reproducing apparatus that carries out recording on and reproduction from a small-sized high-capacity optical disk in order to deal with high-definition still images or moving pictures. A technical method of increasing capacity is to reduce the diameter of a beam spot by reducing the wavelength of a laser beam emitted by an optical pickup apparatus and increasing the numerical aperture (NA) of an objective lens.
The optical disk is generally irradiated with a beam via a transparent optical disk substrate (a cover layer) that covers an information record surface of the disk. With increasing NA, coma is more likely to be caused by a variation in angle between the objective lens and the disk substrate. The causes of the variation in angle include a warp of the optical disk, inclination of a spindle motor that rotates the optical disk, and inclination of an objective lens driving mechanism mounted in the optical head. It is difficult to precisely adjust the angle to accommodate the increased NA, while allowing mass production of this optical disk. Coma resulting from the inclination occurs when a beam passes through the disk substrate, and can thus be lessened by reducing the thickness of the substrate to effect inclination. Accordingly, for an optical disk system using an objective lens with a large NA, an optical disk with a thinner substrate must be used to avoid inclination errors.
On the other hand, the objective lens is designed to form a beam spot with few spherical aberrations on the information record surface of the optical disk when the substrate of a specific optical thickness is used. Consequently, a spherical aberration may occur if the thickness of the substrate deviates from a designed optical thickness. Further, if as in a two-layer disk, two information record surfaces are irradiated with a laser beam from the same direction (compared to a double-sided disk on which recording and reproduction are carried out from different directions), the optical thickness of the transparent layer substrate always differs between the two layers. A spherical aberration caused by an error in the substrate thickness substantially increases consistently with NA. For a lens with a large NA such as 0.85, it is difficult to neglect the effects of an error in the optical thickness of an optical disk manufactured by a normal method.
The term “optical thickness” as used herein refers to a value determined by the thickness and refractive index of an optical disk substrate through which light is transmitted. Optical disk substrates having different thicknesses have an equal optical thickness if beam spots obtained by transmitting light through these substrates have an equal level of spherical aberration. Even if the substrate consists of a plurality of layers, its optical thickness is determined by the thicknesses and refractive indices of these layers.
Jpn. Pat. Appln. KOKAI Publication No. 5-151609 discloses various methods of correcting spherical aberrations caused by a variation in thickness of the optical disk substrate. One of these methods uses what is called a relay lens composed of a convex lens and a concave lens. The relay lens composed of the convex and concave lenses is arranged in front of a position where light emitted by a laser diode is incident on the objective lens. A beam incident on the optical disk through the objective lens has its spherical aberrations varied by varying the position of either the convex or concave lens in the direction of an optical axis. This cancels spherical aberrations that may occur in the optical disk to generate a beam spot with few aberrations on the optical disk. This publication uses a VCM (Voice Coil Motor) as a driving mechanism for the objective lens but does not disclose its structure or a control method therefor.
Jpn. Pat. Appln. KOKAI Publication No. 5-266511 discloses another method somewhat in detail. The method disclosed in this publication uses as relay lens driving means a method of attaching a rack to a relay lens to be moved so that a pinion is rotated to move the lens. This publication also discloses a method of using a measuring instrument provided in a recording and reproducing apparatus to measure a substrate thickness (cover layer thickness), or reading the thickness data recorded in the disk, and setting the position of the relay lens in accordance with the thickness.
Jpn. Pat. Appln. KOKAI Publication No. 2001-28147 also discloses another method somewhat in detail. In this example, the interval between a first and second lenses of the relay lens can be changed using voice coil motors. These voice coil motors are designed so as to vary the interval between the lenses linearly with input currents. To obtain a lens interval suitable for recording in or reproduction from the first or second layer, currents having the same magnitude and flowing in the opposite directions may be allowed to flow through the respective coils.
However, the above described conventional techniques have a plurality of problems.
Jpn. Pat. Appln. KOKAI Publication No. 5-151609 does not provide a detailed description of a driving mechanism using a VCM, as described previously. However, a lens moving mechanism to correct spherical aberrations cannot exhibit sufficient performance with a general simple VCM. Therefore, the technique disclosed in this publication does not allow a practical optical head to be manufactured.
Jpn. Pat. Appln. KOKAI Publication No. 5-266511 discloses an example in which the rack and pinion are used as a relay lens movement device. An optical disk apparatus with an optical head mounted therein is used in a portable computer, a music reproducing apparatus, or the like, and must thus be very small in size. However, with the configuration according to this publication, which has a large number of components including power transmission mechanisms such as a rotary motor and the rack and pinion which convert rotational movement into linear movement, it is difficult to provide a small-sized optical head.
The lens moving mechanism to correct spherical aberrations must move the lens only in the direction of the optical axis. If the lens is inclined or its optical axis deviates from its correct position as the lens is moved, an aberration may occur to hinder the appropriate beam spot from being obtained. Furthermore, if the optical axis deviates from its correct position, the direction of a beam traveling from the relay lens to the objective lens may change. Then, disadvantageously, the position of the beam spot may change. If the position of the beam spot shifts in the radial direction of the disk, then a tracking servo mechanism of an optical drive device moves an objective lens actuator in a tracking direction to prevent the beam spot from deviating from the proper track. However, of course, the acceleration and movement of the beam spot have upper limits. Accordingly, the inclination of the relay lens and the positional deviation of the optical shift must be minimized. None of the conventional techniques deal with this point. Jpn. Pat. Appln. KOKAI Publication No. 5-266511 also fails to describe a mechanism that provides such a function.
Actually, it is expected to be difficult to design a lens moving mechanism using a rack and a pinion, so as to prevent these problems. For example, a typical rack and pinion mechanism involves backlash and is thus readily affected by vibration or a shock. An optical disk apparatus includes vibration inducing components such as a spindle motor rotating the disk and an objective lens actuator even if it is of an installed type instead of a portable type. Further, equipment such as computer in which an optical disk apparatus is mounted often includes a source of vibration such as a fan. With backlash, the apparatus is prone to be affected such vibration. That is, it is difficult to control the position of the relay lens using the rack and pinion mechanism while information is being recorded on or reproduced from the optical disk.
In the VCM disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-28147, the opposite sides of the second lens are supported by plate springs. It is difficult for such a center impeller support structure to establish linear moving directions. Thus, this structure is available only for very short strokes. Accordingly, a sufficient spherical aberration correcting capability is not ensured unless the focal lengths of the first and second lenses are short. In this case, however, the lenses may have to be aspherical, or the ranges of possible inclination of the lens or of permissible errors in eccentricity may be narrowed. As a result, the apparatus cannot be easily assembled. Furthermore, such a support structure is not rigid enough to support the lenses so as to prevent its optical axis from being inclined. Consequently, the lenses may be rotated by a possible vibration or shock, thereby increasing optical aberrations. Further, since the lens interval is controlled on the basis of the amount of current conducted, the effects of a vibration or shock cannot be eliminated. As a result, the lenses may also be moved in the direction of the optical axis by the vibration or the like. This publication cites Jpn. Pat. Appln. KOKAI Publication No. 10-188301 as a conventional example. According to Jpn. Pat. Appln. KOKAI Publication No. 10-188301, spherical aberrations are corrected by varying the interval between two objective lenses using a VCM. With this method, however, after the appropriate current required to correct the interval has been determined, it is maintained to retain the interval. Accordingly, the effects of disturbances cannot be eliminated.
As described above, in a conventional optical head having lenses mounted therein and which can be moved to compensate for spherical aberrations, the proper lens positions cannot be retained. This results in many optical aberrations and makes it difficult to stably control the optical head.
Further, the optical head has an objective lens driving device mounted therein as a lens moving mechanism. Suppression of the positional deviation of the optical axis is not considered for the objective lens driving device because the objective lenses are originally designed to shift the optical axes from a beam for a tracking operation. Furthermore, an objective lens actuator is designed mainly to allow a beam spot to dynamically follow movement of the disk such as decentering or side runout thereof so as to follow frequency components of as low as several kHz. However, a relay lens driving mechanism is designed mainly to correct a spherical aberration caused by a difference between layers of a multilayered optical disk as well as a difference in optical thickness between the transparent substrates of disks; the spherical aberration and thickness difference are not varied by rotation of the disk. A variation in optical thickness of the transparent substrate on the disk surface, which may have to be followed, is small. Consequently, the relay lens driving mechanism mainly perform DC operations and requires a control band of only about several hundred Hz at the most. Thus, the conventional moving devices determine the lens positions using a fixed current or use a mechanism such as a rack and pinion which is not capable of fast reciprocation. Consequently, it is unreasonable to use the objective lens driving apparatus as a relay lens driving mechanism.
Further, with the above described conventional techniques, the relay lens is coaxially arranged on the optical axis of the objective lens, thereby disadvantageously increasing the thickness of the optical head. An optical disk drive with an optical head mounted therein must be thin if it is used as a storage device for a computer, notably a portable computer. Thus, the optical head must not be thick. Obviously, the above described conventional optical heads are thicker than an optical head without any general relay lenses by an amount equal to the thickness of the relay lens. Moreover, even if a mirror is interposed between the objective lens and the relay lens to bend the optical axes through 90°, the driving mechanism used in the conventional examples makes it difficult to reduce the thickness of the optical head.